1. Field of the Invention
The present invention relates to a varifocal zoom lens and a camera system. More specifically, the present invention relates to a varifocal zoom lens including a plurality of movable lens groups for changing focal length, particularly, a varifocal zoom lens suitable for use on a camera system provided with a solid-state imaging device, such as a video camera or a digital still camera, and to a camera system provided with the varifocal zoom lens.
2. Description of the Related Art
A known recording method of recording an image of an object records the image of the object by converting the quantity of light forming the image formed on the surface of a camera system using, as a recording means for a camera, a photoelectric converter, such as a CCD (charge-coupled device) or a CMOS (complementary metal-oxide semiconductor), into electric signals by the photoelectric converter.
The operating speed of central processing units (CPUs) has been increased and the degree of integration of storage mediums has been increased with the recent advancement of fine-processing techniques. Consequently, high-speed processing of image data of a large capacity, which had been difficult, has been realized. The scale of integration of light-receiving devices has been increased and the miniaturization of light-receiving devices has advanced. The increase in the scale of integration made recording at higher spatial frequencies possible, and the miniaturization of the light-receiving device brought about the miniaturization of cameras.
The increase in the scale of integration and miniaturization of the light-receiving device reduced the light-receiving area of each photoelectric conversion element. Consequently, the magnitude of an electric output provided by each photoelectric conversion element is reduced, and hence the effect of noise has become serious. An attempt made to prevent the adverse effect of noise placed a small lens unit, namely, a microlens array, right in front of photoelectric conversion elements to increase the quantity of light falling on the light-receiving elements by increasing the aperture ratio of the optical system. The microlens array placed restrictions on the position of the exit pupil of the lens system instead of guiding light rays falling on boundaries between the adjacent elements onto the elements. When the exit pupil of the lens system approaches the light receiving elements, i.e., when an angle between the principal ray falling on the light-receiving element and the optical axis increases, off-axis light rays falling on a peripheral part of an image surface are inclined at large angles to the optical axis and, consequently, the off-axis light rays are unable to fall on the light-receiving elements and an insufficient quantity of light falls on the light receiving elements.
Whereas a camera provided with a photoelectric converter as a light-receiving device, namely, a digital still camera, does not need any developing operation and facilitates data handling, such as the confirmation of an image formed by the camera, the image quality of the image is inferior to that of an image formed on a 35 mm film format camera using a 35mm film as a recording medium, and the camera needs to be connected to equipment, such as a computer. Thus the number of digital still cameras in use has not increased. Digital still cameras have become generally used in recent years owing to the improvement of image quality and the diffusion of equipment.
Both an increase in the scale of integration of the light-receiving device and an improvement of the performance of the optical system are important for the improvement of image quality.
An increased zoom ratio increases the user's degree of freedom of phototaking. For example, an increase zoom ratio enables phototaking at a short object distance and phototakingg scenery in a wide range in a room at a short object distance.
A known practical zoom lens is the so-called negative-negative-positive three-group lens including, for example, a first lens group having a negative refractive power, a second lens group having a positive refractive power and a third lens group having a positive refractive power arranged from the side of the object in that order.
Zoom lenses are mentioned in Patent documents 1, 2, 3 and 4.
The zoom lens mentioned in Patent document 1 employs an Ernostar type lens formed by combining four lenses, namely, convex, convex, concave and convex lenses, as a second lens group. The zoom lens mentioned in Patent document 2 employs an Ernostar type lens formed by combining a second lens group including three lenses, namely, convex, convex and concave lenses, and a third lens group. The zoom lens mentioned in Patent document 3 employs a triplet type lens formed by combining three lenses, namely, convex, concave and convex lenses, as a second lens group. The zoom lens in a fifth embodiment according to the invention disclosed in Patent document 4 has a second lens group including a convex/concave cemented lens and a convex lens.
The light-receiving device has been miniaturized through the increase of the scale of integration in recent years, and it has been desired to achieve both the miniaturization of the lens system and the improvement of the performance of the lens system. It has been desired to suppress the deterioration of the performance of the lens system due to the decentration of the component lenses caused during the manufacturing process.
Patent document 1: JP-A 2003-66332
Patent document 2: JP-A 2003-140041
Patent document 3: JP-A 2003-140047
Patent document 4: JP-A 2003-149555